bkkeepr is a web application that uses Twitter to track what people are reading.
bkkeepr records peoples current reading and bookmarks, using a number of public APIs to gather book metadata including covers. These are then made public and shareable, and allow further bridging to bookshops and social networks.
bkkeepr was built by booktwo.org as part of our ongoing investigations into literature and technology. It was built entirely from open-source software, and launched in May 2007. It was supported by sponsorship in the form of advertising from the outset.
More information in this post.